The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or on a retroreflector target in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. Coordinate-measuring devices closely related to the laser tracker are the laser scanner and the total station. The laser scanner steps one or more laser beams to points on a surface. It picks up light scattered from the surface and from this light determines the distance and two angles to each point. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. Hereinafter, the term laser tracker is used in a broad sense to include laser scanners and total stations.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of laser tracker contains only an interferometer (IFM) without an absolute distance meter (ADM). If an object blocks the path of the laser beam from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner, as described in more detail below. Some laser trackers contain only an ADM without an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges et al., the contents of which are herein incorporated by reference, describes a laser tracker having only an ADM (and no IFM) that is able to accurately scan a moving target. Prior to the '446 patent, absolute distance meters were too slow to accurately find the position of a moving target.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest.
Angle measuring devices such as angular encoders are attached to the mechanical axes of the tracker. The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
Several laser trackers are available or have been proposed for measuring six, rather than the ordinary three, degrees of freedom. Exemplary six degree-of-freedom (six-DOF) systems are described by U.S. Pat. No. 7,800,758 ('758) to Bridges et al., the contents of which are herein incorporated by reference, and U.S. Published Patent Application No. 2010/0128259 to Bridges et al., the contents of which are herein incorporated by reference.
One type of ADM in use today determines the distance to a target by measuring the shift in phase of a sinusoidally modulated beam of light as the beam travels from the measurement device to a target and back. To measure the shift in phase of the light, the detected light is downconverted using one or more mixers and then sent to an analog-to-digital converter (ADC) to obtain measurement samples that are processed to determine the phase. In this scheme, the mixer adds complexity and cost to the ADM design, and therefore it would be better if it were eliminated. Another problem with the use of mixers in a downconversion stage is that a mixer may undergo a shift in phase with variation in the power level of an RF signal entering the mixer, thereby producing an error in the calculated distance to the measured target.
The paper “Digital laser range finder: phase-shift estimation by undersampling technique” by Poujouly et al., herein incorporated by reference, describes two methods for extracting phase in a phase-based distance meter. In the first method, a quadrature (I/Q) demodulation scheme is used in conjunction with digital filters and an automatic gain control (AGC). The accuracy obtained with the disclosed method is approximately 6 mm, which is about a factor of 1000 worse than that desired for the application considered herein. In the second method, a first frequency is used to modulate a laser. The modulated laser signal is sent to a target, and the detected signal is sampled at another frequency in an ADC to obtain sample values that can be used to calculate the phase shift of the modulated light. However, a single frequency is not sufficient to operate over a relatively large range since multiple modulation frequencies are required to determine the “unambiguity region” in which a measured target resides. The architecture disclosed does not permit this ambiguity to be removed.
U.S. Pat. No. 7,177,014 ('014) to Mori et al. discloses a method for measuring a distance to an object using an absolute distance meter based on an undersampling method. The method in this patent applies a first or a second modulation signal to modulate the optical power of a laser. The detected light is applied to a first ADC while an electrical signal of the same frequency is applied to a second ADC. The phase difference between the two ADC signals is used to calculate the distance to the object. However, the disclosed method does not provide good rejection of noise coming from the laser diode and the optical detectors because the second ADC signal is electrical only. Because the Mori patent does not disclose a distance meter used with a retroreflector but rather with objects such as [from the '014 patent] “an aluminum plate, a card board, a PC plate, a velvet cloth, a black paper, wood, and a painted plate,” the distance accuracies are much lower than those possible with a retroreflector. For dimensional measurement systems in which a retroreflector is used, there is generally a need for much higher accuracies, and in these cases it is important that an optical reference signal be provided to remove common mode noise associated with the laser and optical detectors. In addition, the methods the '014 patent provides for resolving range ambiguities are restrictive. In most practical systems requiring relatively high accuracy, it is necessary to provide a way to apply three or more modulation frequencies to remove ambiguity.
What is needed is a relatively inexpensive method for obtaining relatively high accuracy absolute distance measurements.